1. Field of the Invention
This invention relates to a method of making a magnetic recording medium. This invention particularly relates to a method of making a magnetic recording medium which exhibits excellent orientation of magnetic grains and excellent surface smoothness.
2. Description of the Prior Art
Recently, a strong need exists for improvement of the S/N ratio of magnetic recording media. In order to satisfy the need, various attempts have been made to improve the S/N ratio by decreasing the size of magnetic grains constituting magnetic layers of magnetic recording media or by improving the surface smoothness of magnetic layers.
As one type of generally used magnetic recording media, there has heretofore been known a magnetic recording medium comprising a non-magnetic substrate and a magnetic layer overlaid on the surface of the non-magnetic substrate by the application of a magnetic coating composition containing a binder dissolved in a solvent and magnetic grains dispersed therein. The magnetic recording medium of this type is made by continuously moving the non-magnetic substrate, by applying the magnetic coating composition onto the surface of the non-magnetic substrate, and by drying and hardening the coating film thus applied to form the magnetic layer. In order to improve the squareness of the magnetic hysteresis curve of the magnetic layer, an external magnetic field is applied to the coating film before it is hardened, thereby orientating the easily magnetizable axis of magnetic grains in a desired direction.
It is generally known that orientation of the magnetic grains orientated by the magnetic orientation processing tends to be disturbed before the coating film is dried. Experiments conducted by the inventors revealed that the tendency toward orientation disturbance increases as the grain size of the magnetic grains is decreased. Therefore, the extent to which the grain size of the magnetic grains can be decreased for the purpose of improving the S/N ratio as described above is limited.
Accordingly, in order to improve the S/N ratio, it becomes essential to improve the surface smoothness of the magnetic recording medium. However, the surface smoothness is also closely related to the orientation of the magnetic grains. That is, when a magnetic orientation processing is carried out for orientating the magnetic grains, the magnetic grains agglomerate and roughen the surface of the coating film to deteriorate the surface smoothness. In order to eliminate this problem, it has been proposed in Japanese Patent Publication No. 56(1981)-36496 to preliminarily dry the coating film to such an extent that the magnetic grains are not magnetically agglomerated by an orientation magnetic field and then to carry out an orientation processing and drying. However, with this method, it is not possible to sufficiently improve the orientation of the magnetic grains.